The present invention relates to an electric control arrangement for the speed control of a rolling mill drive motor of a rolling mill for producing hollow blocks, such as is known in principle in rolling mill technology from DE-OS No. 23 26 115.
Such hollow blocks, also known in the industry as blooms, are the basic material for a subsequent rolling process which stretches the hollow block into a finished or semifinished tube. For producing such a hollow block, a steel block is rolled over a mandrel by means of the so-called diagonal rolling process or Mannesmann process. The diagonal rolling process is a hot rolling process in which a hot steel block is set in rotary motion with axial feed between mutually inclined rolls having the same direction of rotation. The block is held between the rolls by means of guide rolls, for example. Approximately in the central axis of this system, a mandrel is arranged which extends with its tip into the rolling gap approximately to one-half the roll length. In this manner the block is rolled over the mandrel in the course of the rolling process so that at the end, a hollow block or cylinder is produced which serves as the starting material for the subsequent tube rolling. A disadvantage of this process is that radial cracks are produced when the cavity is formed in front of the tip of the mandrel. If the cracks exceed a certain extent, the bloom must be scrapped. With fixed nominal speed values, the crack formation can assume an unpermissible extent if the material properties are changed due to temperature, a change of charges or other inhomogeneities.